Equilibria in Elemental Crystals

In elemental crystals the native defects are vacancies and interstitials. Vacancies may be formed by removing atoms from normal sites and placing them on the surface according to the reaction 

However, the addition of atoms to the surface does not increase 

Interstitial defects can be formed by placing surface atoms into interstices  

The removal of an atom from the surface essentially leaves the total number of surface atoms constant because interior atoms then become surface atoms. However, the total number of atoms on normal sites decreases and Eq. (11) should be written 

The total number of sites also decreases with formation of inter- 

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